Jasleen Lugani

Generation of polarization-entangled photons using type-II doubly periodically poled lithium niobate waveguides

In this paper, we address the issue of the generation of nondegenerate cross-polarization-entangled photon pairs using type-II periodically poled lithium niobate. We show that, by an appropriate engineering of the quasi-phase-matching grating, it is possible to simultaneously satisfy the conditions for two spontaneous parametric down-conversion processes, namely, ordinary pump photon down conversion to either extraordinary signal and ordinary idler paired photons or to ordinary signal and extraordinary idler paired photons. In contrast to single type-II phase matching, these two processes, when enabled together, can lead to the direct production of cross-polarization-entangled states for nondegenerate signal and idler wavelengths. Such a scheme should be of great interest in applications requiring polarization-entangled nondegenerate paired photons with, for instance, one of the entangled photons at an appropriate wavelength being used for local operation or for quantum storage in an atomic ensemble and the other one at the typical wavelength of 1550 nm for propagation through an optical fiber.

Generation of modal- and path-entangled photons using a domain-engineered integrated optical waveguide device

Integrated optical devices are expected to play a promising role in the field of quantum information science and technology. In this paper we propose a scheme for the generation of nondegenerate, copolarized, modal, and path-entangled photons using a directional coupler and an asymmetric Y-coupler geometry in type-0 phase-matched domain-engineered lithium niobate (LN) waveguide. The nonlinearity in LN is tailored in such a way that quasi-phase-matching conditions for two different spontaneous parametric down-conversion processes are obeyed simultaneously, leading to a modal and path-entangled state at the output. Assuming typical values of various parameters, we show, through numerical simulations, that an almost maximally entangled state is achievable over a wide range of waveguide parameters. For the degenerate case, the proposed scheme gives a NOON state for N = 2. The generated entangled photon pairs should have potential applications in quantum information schemes and also in quantum metrology. By appropriate domain engineering and component designing, the idea can be further extended to generate hyperentangled and two-photon multipath-entangled states, which may have further applications in quantum computation protocols.

Generation and characterization of discrete spatial entanglement in multimode nonlinear waveguides

We analyze theoretically spontaneous parametric down-conversion in a multimode nonlinear waveguide as a source of entangled pairs of spatial qubits, realized as superpositions of a photon in two orthogonal transverse modes of the waveguide. It is shown that by exploiting intermodal dispersion, down-conversion into the relevant pairs of spatial modes can be selected by spectral filtering, which also provides means to fine-tune the properties of the generated entangled state. We also discuss an inverting interferometer detecting the spatial parity of the input beam as a versatile tool to characterize properties of the generated state. A single-photon Wigner function obtained by a scan of the displaced parity can be used to identify the basis modes of spatial qubit, whereas correlations between displaced parity measurements on two photons can directly verify quantum entanglement through a violation of Bells inequalities.

Fock-space exploration by angle-resolved transmission through a quantum diffraction grating of cold atoms in an optical lattice

Light transmission or diffraction from different quantum phases of cold atoms in an optical lattice has recently come up as a useful tool to probe such ultracold atomic systems. The periodic nature of the optical lattice potential closely resembles the structure of a diffraction grating in real space but is loaded with a strongly correlated quantum many-body state which interacts with the incident electromagnetic wave, a feature that controls the nature of light transmission or dispersion through such quantum media. In this paper we show that, as one varies the relative angle between the cavity mode and the optical lattice, the peak of the transmission spectrum through such cavities also changes, which reflects the statistical distribution of the atoms in the illuminated sites. Consequently, the angle-resolved transmission spectrum of such quantum diffraction gratings can provide a plethora of information about the Fock-space structure of the many-body quantum state of ultracold atoms in such an optical cavity that can be explored in current state-of-the-art experiments.

Discrete spatial entanglement in multimode nonlinear waveguides (Conference Presentation)

We present a scheme for generation and characterization of entangled spatial qubits based on type-II spontaneous parametric down-conversion (SPDC) in a periodically poled titanyl phosphate (PPKTP) multimode nonlinear waveguide [1]. Our scheme exploits intermodal dispersion which has been hitherto successfully employed to produce spatially pure SPDC photon pairs from a multimode waveguide without spatial filtering [2]. Production of discrete entanglement relies on driving simultaneously two SPDC processes that involve different combinations of transverse spatial modes for which phase matching bandwidths significantly overlap. We propose a procedure for experimental identification of the spatial qubit subspace based on a scan of the spatial Wigner function via the displaced parity measurement using an inverting Sagnac interferometer and photon counting. We numerically verified the robustness of the mode reconstruction procedure against experimental imperfections. We also propose an experimental method for detecting spatial entanglement in the position-wave vector phase space. Numerical simulations indicate that waveguide parameters required for experimental demonstrations are compatible with current manufacturing capabilities. Using simulated mode profiles we calculate the maximum attainable Clauser-Horne- Shimony-Holt combination value reaching 2.12, which clearly violates the classical limit and confirms the feasibility of observing non-classical features of the generated state. [1] M. Jachura et al. Physical Review A, 95, 032322 (2017). [2] M. Jachura, M. Karpinski, C. Radzewicz, K. Banaszek, Optics Express, 22, 8624-8632 (2014).

Phase sensitive amplification enabled by coherent population trapping

We isolate a novel four-wave mixing process, enabled by Coherent Population Trapping (CPT), leading to efficient phase sensitive amplification. This process is permitted by the exploitation of two transitions starting from the same twofold degenerate ground state. One of the transitions is used for CPT, defining bright and dark states from which ultra intense four-wave mixing is obtained via the other transition. This leads to the measurement of a strong phase sensitive gain even for low optical densities and out-of-resonance excitation. The enhancement of four-wave mixing is interpreted in the framework of the dark-state polariton formalism.

Time-dependent phase shift of a retrieved pulse in off-resonant electromagnetically-induced-transparency–based light storage

We report measurements of the time-dependent phases of the leak and retrieved pulses obtained in EIT storage experiments with metastable helium vapor at room temperature. In particular, we investigate the influence of the optical detuning at two-photon resonance, and provide numerical simulations of the full dynamical Maxwell-Bloch equations, which allow us to account for the experimental results.We report the experimental observation of a Coherent Population Oscillation (CPO) based light storage in an atomic vapor cell at room temperature. Using the ultranarrow CPOs between the ground levels of a Λ system selected by polarization in metastable He, such a light storage is experimentally shown to be phase preserving. As it does not involve any atomic coherences it has the advantage of being robust to dephasing effects such as small magnetic field inhomogeneities. The storage time is limited by the population lifetime of the ground states of the Λ system.

Identifying insulating states of ultra cold atoms with cavity transmission spectrum

In this paper, we consider the transmission characteristics of an optical cavity loaded with ultracold atoms in a one-dimensional optical lattice at absolute zero temperature. In particular, we consider the situation when the many body quantum state of the ultracold atoms is an insulating state with a fixed number of atoms at each site, which can either be density wave or Mott insulator phase, each showing a different type of discrete lattice translational symmetry. We provide a general framework of understanding the transmission spectrum from a single and two cavities/modes loaded with such insulating phases. Further, we also discuss how such a transmission spectrum changes when these insulating phases make a cross over to the superfluid phase with the changing depth of the optical lattice potential.

Switchable hyperentangled photon pairs from an integrated optic waveguide device

In this paper, we propose an integrated optic waveguide device employing a modified Mach–Zehnder interferometer, capable of generating nondegenerate, hyperentangled photon pairs. The geometry enables multiple (eight) type-II phase-matched spontaneous parametric downconversion processes simultaneously, resulting in a biphoton state, which is simultaneously entangled in polarization and spatial modes. Using an electro-optic phase modulator, we show the possibility of altering modal entanglement without affecting polarization entanglement. Such switchable, maximally entangled photon pairs, entangled in multiple degrees of freedom, should be very useful in various on-chip quantum optics experiments and in the implementation of quantum information protocols employing higher dimensional entanglement.

Hyper-entangled photon pairs using integrated optical waveguide device

We propose a scheme for the generation of hyper-entangled photon pairs using an integrated optical waveguide device; the biphoton output state is simultaneously entangled in polarization and spatial modal degrees of freedom.